Self-adhesive compositions foamed by means of expandable microspheres are known. Compared to open-celled foamed self-adhesive compositions, they achieve better sealing against dust and liquids due to their closed cells. Compared to unfoamed self-adhesive compositions they adapt better to the roughness of a substrate to be adhesively bonded. Compared to self-adhesive compositions foamed by means of hollow glass spheres, they are more deformable and better equalize thermal expansion differences between various adhesively bonded materials in the case of fluctuating temperatures and damp vibrations and insulate against vibrations better.
The thickness of the shell of the microsphere and the thermoplastic polymer of which it consists influence the mechanical properties of the foam just as much as the unexpanded diameter of the microsphere and the type of blowing agent present in the interior thereof. These parameters also influence the expansion start temperature, the expansion rate, the maximum expansion capability and the robustness of the microsphere.
Many types of expandable microspheres are commercially available; preference is given to using unexpanded types for the purposes of the invention because these have a greater expansion capability than the previously expanded types. Within the class of unexpanded microspheres, there are many types from various manufacturers; they differ essentially in their diameter (from 6 to 9 μm in the case of smallest types of Expancel, namely 461 DU 20, and from 28 to 38 μm in the case of the largest types of Expancel, namely 920-, 930-, and 951 DU 120) and their temperatures required for starting expansion (from 75 to 220° C.). One example of commercially available microspheres are the Expancel® DU grades (DU=dry unexpanded) from Akzo Nobel.
The word “uniform” (=“homogeneous”) is used in the present application in the sense that the specific mass of a test specimen taken from the product, whose edge length naturally has to be substantially greater (i.e. at least about 30 times as large) than the diameter of the largest expandable microsphere therein, is virtually independent of where in the product this test specimen is taken. This word therefore does not mean, for instance, that all expanded microspheres have to have the same size; they could not have the same size because their precursors, the unexpanded microspheres, are produced in a random process and therefore have a certain fluctuation from individual microsphere to individual microsphere both in terms of their diameter and in their wall thickness.
The production of expandable microspheres is described in principle in U.S. Pat. No. 3,615,972 by Dow.
DE 21 05 877 C discloses a self-adhesive tape having a support which is coated on at least one side with a microcellular self-adhesive composition (also referred to as “pressure-sensitive adhesive”). This adhesive layer contains a nucleating agent, and the cells of the adhesive layer are closed and distributed “uniformly” in the adhesive layer.
U.S. Pat. Nos. 6,103,152 and 6,797,371, which correspond to EP 1 102 809 B1, teach a process for producing polymer foam by means of expandable microspheres. In a public nullity action against the German part of the European patent, the patent proprietor has stated that the expression “commencement of foaming before leaving the die”, which from the context obviously means the shaping die at the end of a second extruder, is actually intended to mean commencement of foaming in the die. The German Federal Court has followed this in its decision “Polymerschaum” of 17 Jul. 2012 (File no.: XZR 117/11, paragraph 31, last sentence).
EP 0 257 984 A1 describes self-adhesive tapes which have a foamed adhesive layer containing microspheres on at least one side. According to page 3, lines 38 to 41, the microspheres can be mixed in their unexpanded state into the adhesive composition and expanded by means of later heating, or, preferably, be expanded first and then mixed into the adhesive composition.
The further documents DE 35 37 433 A1, WO 95/31225 A1, WO 95/32851 A1, EP 0 693 097 A1, WO 98/18878 A1 and DE 197 30 854 A1 appear to be less relevant.
DE 10 2008 004 388 A1 teaches a process for producing an unfoamed self-adhesive composition using natural rubber as base polymer. In these contexts, examples 5 and 6 on pages 23 and 24 teach a production process using, inter alia, a planetary-gear extruder. When the “heating zones” there are indicated to be the two radially outermost, standing hollow gears of a two-module planetary-gear extruder, it is proposed there that the temperature of the hollow gear is maintained at 50° C. in both modules, while the sun gear (=central spindle) is maintained at only 10° C.